The need for organic surface modification arises from the desire to develop materials capable of serving multiple functions beyond their native properties. For example, while one may require the mechanical strength-to-weight ratio and corrosion resistance of stainless steel to build reactors in the cosmetics industry, but may also need the surface of the reactor walls to repel the often sticky cosmetic formulations—properties that are not native to the base material—while ensuring that the cosmetic product is not in any way contaminated.
In the field of nanomaterials, the exceptional conductivity of carbon nanotubes makes them ideal materials to form conductive nanocomposites for the aeronautics industry; at the same time, their homogeneous dispersion and appropriate distribution in a polymer matrix requires that they be imparted with a significant surface charge and functional groups that are stable over a wide range of processing temperatures and compatible with the host matrix. Further, surface treatment for nanoparticles may also be required to avoid agglomeration.
Surface modification can currently be achieved through two main streams: adsorption and functionalization.
Surface adsorption is the simplest method to impart a charge or steric hindrance to a surface by using compounds known as surfactants. These are widely used in the field of colloids to promote dispersion of small particles in a host fluid. On macroscopic surfaces, these compounds can also be used to form self-assembled monolayers (SAMS) that can, for example, alter the wettability of a surface. Various functional moieties can also be applied using SAMS. However, the basic functionalities achievable through surface adsorption face a severe limitation: the coating is not bonded to the surface and is thus prone to thermal or mechanical desorption (surfactants are known to desorb at temperatures as low as 70° C.).
On the other hand, functionalization allows for the formation of a strong covalent bond between the functional moiety and the substrate. Functionalization can be achieved through solvent-based chemistry or gas-phase deposition.
The currently favoured liquid-based methods can be fairly problematic: achieving the desired functionality requires knowledge about the specific reactions or reaction mechanisms (often complex and multi-step, and involving potentially toxic solvents and reactants). Moreover, it can be quite difficult to identify the appropriate medium through which to conduct the functionalization reactions (the substrates and the functionalization reagents may not be compatible with the same solvent). Furthermore, in the case of nanoparticle functionalization, the separation of the functionalized particles from the leftover reagents, undesirable by-products and solvents typically requires significant downstream processing, thereby leading to efficiency loss, which limits the potential for scale-up and increases the overall cost of treatment. These difficulties are compounded when attempting to form multi-functional or “smart” surfaces, given the increased number of reagents and possible products involved. These methodological shortcomings are generally dodged in the literature.
Solvent-free gas-phase methods, typically referred to as chemical vapour deposition (CVD), do not face these particular issues: gases are miscible (at normal pressure) and readily separate from solid substrates. In a typical CVD process, a substrate is exposed to one or more volatile precursors (gas), which react and/or decompose on the substrate surface to produce a coating. The advantage of this technique is that it allows rapid deposition of a consistent and clean coating. CVD is typically stimulated or initiated by one (or a combination) of three energy sources: heat, plasma or light.
Thermally activated CVD (TACVD) is mostly reserved to inorganic coatings, as the high temperatures required to achieve the desired reaction activation energy are incompatible with most organic compounds. This limitation can be curtailed in some cases through the use of initiator compounds.
Plasma-enhanced CVD (PECVD) allows for the creation of a non-thermal highly reactive environment through ionization, which leads to strong electron, ion and UV light bombardment of organic species. PECVD has been extensively used to form thin organic coatings (often referred to as “plasma polymerization”). The possibility of using PECVD for tailoring the wettability of surfaces by adjusting the process parameters of plasma enhanced CVD has been demonstrated. It was also shown that UV light contributes to PECVD's efficiency. While this technique is successful for organic surface functionalization, it suffers from a processing point of view. Indeed, it requires specialized equipment and, in most cases, that processing occurs under low pressure, thus limiting treatment volumes and throughput. Moreover, the use of certain electronegative compounds, such as oxygen, can rob PECVD of its efficiency. This technique is therefore best suited for high value-added applications.
Photo-initiated CVD (PICVD) allows for a decoupling of the useful components of plasma processing (such as UV radiation and use of small organic precursor compounds) from the process itself. Thus, specific plasma processing conditions, such as operating under vacuum, can be avoided. Indeed, UV lamps (typically glow discharge plasmas) are separated from the process by a UV-transparent window, thereby allowing for the functionalization process to operate under atmospheric or near-atmospheric conditions. Photochemical reactions have been used to grow SiO2 layers in the semi-conductor industry and to deposit certain organic coatings on macroscopic surfaces.
These processes typically resort to high-energy, low-wavelength vacuum UV (VUV, <200 nm) or extreme UV (EUV, <121 nm) sources such as D2, Hg or excimer lamps, or custom-made plasma sources. In fact, PICVD efforts have almost all relied on the use of such VUV and EUV sources. This is due to the fact that, at such low wavelengths, it is possible to specifically target certain molecular bonds and break them. This favors large coating thickness and fast deposition kinetics, which are the main focus of the semi-conductor industry, from which most PICVD studies arise. In fact, there is an established consensus in the PICVD literature that it is necessary to use a light source emitting radiation close to the peak absorption of the gas precursor used for PICVD to be useful. Therefore, light sources are chosen so as to emit at a wavelength at which the gas precursor used exhibits a significant absorption cross-section. However, it should be noted that VUV and EUV sources can be very costly. In addition, they require the use of specific window materials to allow for light transmission. In fact, MgF2, LiF, and CaF2 are the only common materials with significant transparency below 200 nm, and these expensive materials are fragile and particularly prone to chemical attack, which complicates VUV and EUV PICVD. In the absence of a light source with the appropriate emission wavelength, the literature teaches, in some cases, to use a photosensitizer compound sensitive to the wavelength of the light source. See references 1-5 in the section entitled “References” below.
PICVD has only sparsely been used for the coating of nanomaterials, but it has been shown to have potential as a gas phase nanoparticle treatment.
On another subject, syngas (also called synthesis gas) is a fuel gas mixture consisting primarily of hydrogen, carbon monoxide, and very often some carbon dioxide. It is combustible, but has less than half the energy density of natural gas. Syngas is a product of several processes, including steam reforming and waste destruction processes such as gasification. Generally, syngas is converted into hydrocarbons or alcohols via various catalytic processes or is burned in a turbine to produce energy (with average to low efficiency). The most common catalytic pathway is the Fischer-Tropsch process.
On yet another subject, UVC light sources have been used in the field of photochemistry to stimulate polymerization reactions (curing), to degrade harmful organics in wastewater (photocatalysis), for lithography and, recently, for nanomaterials synthesis.
On a last subject, nanoparticles, and nanomaterials more generally, are used in scientific fields such as optical and biomedical applications. These applications generally use expensive advanced materials of controlled size and composition. Some other applications would however benefit from cheaper sources of such materials. Nevertheless, by their very nature, nearly all raw nanomaterials need to be surface functionalized prior to incorporation into matrices for use in applications.
In the hope of finding cheaper sources of nanomaterials, attention has recently been paid to unconventional sources of ultrafine powders, such as ash from municipal solid waste (MSW), coal, cane and oil shale. Such interest is mainly justified because the accumulation of large quantities of ash is becoming a serious environmental problem. Both MSW and fly ash are considered as renewable resources. Fly ash is currently disposed of in landfills or used in cements. However, it should be noted that MSW fly ash has a variable composition. Indeed, the composition can vary according to the source of the ash, but typical compositions are silica (49-64% wt), alumina (14-30% wt), iron oxide (6-23% wt) and CaO (1-7% wt). On the other hand, MSW fly ash may contain some valuable materials including As, Al, B, Ba, Cd, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sb, Se, Zn, in oxidized or ionic salt form.